Coreless transformers are transformers that do not have a transformer core. Such coreless transformers can be integrated in or on a semiconductor chip or on a printed circuit board (PCB). These transformers can, therefore, be realized in a space-saving manner. Such transformers can be used in circuit applications in which data or electrical energy is to be transmitted across a potential barrier between two circuits that have different reference potentials. Such a circuit is, for example, a gate drive circuit of a high-side power semiconductor switch, like a MOSFET or an IGBT.
Coreless transformers have a maximum impedance frequency (MIF), which is the frequency for which the transformer has its highest input impedance, and have a maximum efficiency frequency (MEF), which is the frequency for which the transformer has its lowest transmission losses. In particular, when power is to be transmitted using a coreless transformer it is desired to operate the transformer at its, or at least close to its MEF. For a given load scenario MEF and MIF are different from each other, with a difference between MEF and MIF becoming larger with increasing load current.
Transmission properties of a coreless transformer and, therefore, MEF and MIF depend on a number of electrical parameters which, inter alia, include: inductivities of the transformer's primary and secondary windings; ohmic resistances of the transformer's primary and secondary windings; input and output capacitances of the transformer; and an inductive coupling between the transformer's primary and secondary windings. These parameters, due to process variations, may vary even for those transformers that are produced using identical process steps.